The journalist India Knight has been busy plugging her new book In Her Prime: Older, Wiser, Happier. It’s full of Knight’s expert advice on pressing issues such as ‘using blusher’ or the ‘rule’ for eye shadows and liners – something involving the colour wheel apparently. All harmless enough, until she chose to reveal her rather naive views on infidelity on Woman’s Hour. “Affairs? So what?” she glibly pronounced. “So a lot”, shouted every listener who has ever known the pain of betrayal. This one’s for you India.

1. Most people think India Knight’s views on infidelity are a crock. Britain could hardly be described as puritanical, but results from the most recent UK National Survey of Sexual Attitudes and Lifestyle (NATSAL 2013) show that 63% of men and 70% of women disapprove of non-exclusivity in marriage. Ten years ago those figures were 45% for men and 53% for women.

2. It’s easy to be objective when you are thinking rationally, but betrayal, lies and secrecy are a complete head f***. The betrayed partner feels a maelstrom of emotions including depression, rage, abandonment and rejection. Their self-esteem falls to pieces. They lose all their self-confidence and many exhibit symptoms of post-traumatic stress disorder (Cano and O’Leary, 2000, Glass, 2002, Schneider et al.,1998).

3. Because feelings run so high, couple therapists view extra marital affairs as one of the most damaging relationship events and one of the most difficult problems to treat in couple therapy (Whisman, Dixon, & Johnson, 1997).

4. And infidelity is the most commonly cited cause of divorce (Amato & Rogers, 1997). In fact, it is the single most common cause of conjugal dissolution across 160 societies (Betzig, 1989).

5. Infidelity creates a self-perpetuating shit storm. One partner’s affair often spurs the other partner to engage in a revenge affair (Lusterman, 1998). Even the suspicion of a partner cheating is associated with an increased likelihood of the betrayed partner engaging in infidelity.

6. Fortunately, the perception that infidelity is widespread is incorrect. Figures from the NATSAL 2000 study found that just 5% of married women and 10% of married men had had sex with someone other than their spouse in the previous five years. US research by Laumann et al., (1994) found that the prevalence of extramarital sexual intercourse measured over the preceding year was less than 4%.

7. Married men and women who try to pass off a one night stand as an ‘opportunistic shag’ are often in denial about the state of their primary relationship. There is a wealth of research to support the relationship between infidelity and marital dissatisfaction (Atkins, Baucom, & Jacobson, 2001, Buunk, 1980,Treas & Giesen, 2000).

8. It’s a bit of a chicken and egg situation though, because a 17 year study of extramarital sex and marital happiness has found that infidelity is both a cause and a consequence of marital discord (Previti & Amato, 2004).

9. People kid themselves that ‘no one gets hurt if no one finds out’, but that’s a crock too. Affairs change people. From their physical absence to their emotional withdrawal, secrecy is toxic for their spouse and their children. A longitudinal study which assessed couples who had been in therapy once every six months over a five year period, found that only 20% of couples where infidelity had been kept ‘secret’ were still married after five years compared to more than half (57%) of the couples where infidelity had been revealed. In contrast 77% of non-infidelity couples were still together. (Marín, Christensen &Atkins, 2014)

10. And of course, a person who has been unfaithful once will be unfaithful again. Affairs are addictive. Dopamine, the neurotransmitter which motivates us to ‘want’ sex, is stimulated by unpredictability (Berridge and Robinson 1998) and because the neurological ‘wanting’ system (dopamine) is so much more powerful than the ‘satiety’ system (opoids), the uncertainty of extra marital sex, coupled with the risk of losing everything, only increases the anticipation and arousal. The intensity of those feelings leaves a lot of infidels concluding that they must be “in love”, but estimates on the number of affairs that successfully translate into marriage range from 3% (Halper) to 10% (Staheli). So what.